Unfortunately it's proprietary, because of unclear license terms about distribution of source code and modifications (saying that you cannot charge a "copyright license fee") and it clearly says that patentable ideas in the source code are not licensed.
There's a built-in ffmpeg AAC encoder too, but despite the author's claims it is not as good as libfdk_aac.
And libfdk_aac itself isn't as good as Apple's AAC codec either. There's a Hydrogenaudio listening test demonstrating that.